1. Field of the Invention
This invention relates to an anti-skid control apparatus for a vehicle braking system which can prevent locking of the wheels.
2. Description of the Prior Art
An anti-skid control apparatus for a vehicle braking system is known that includes a fluid pressure control valve device arranged between a master cylinder and a wheel cylinder of a brake for the wheel, the fluid pressure control valve device receiving control signals from a control unit measuring the skid condition of the wheel to control the brake fluid pressure to the wheel cylinder.
When a fluid pressure control valve device is provided for each of four wheels, and the fluid pressures to the wheels are independently controlled, there is no problem of control operation. When a fluid pressure control valve device is provided for each of the front wheels, and for both of the rear wheels in common, there is no problem in controlling braking. In the latter case, the one common fluid pressure control valve device is controlled on the basis of the lower one of the speeds of the rear wheels.
However, in the above cases, three or four fluid pressure control valve devices are used. Accordingly, the whole anti-skid control apparatus is large in size and very heavy. Siince the fluid pressure control valve device is expensive, the cost is relatively high.
For example, suppose the brake fluid pressures of the front wheels are controlled by the fluid pressure control valve devices respectively in a diagonal or X-type conduit system, and the brake fluid pressures of the rear wheels are controlled in common with the front wheels. When the vehicle runs on the road in which the right and left sides of which have considerably different coefficients of friction, the one rear wheel located diagonally to the one front wheel on the higher frictional coefficient side is locked. In that case, the steering of the vehicle becomes unstable, a condition that is very dangerous.
Further, even if proportioning valves are provided for the rear wheels, the brake fluid pressure of the rear wheels increase in proportion of the input fluid pressures to the proportioning valves. There is still a danger of locking.
Accordingly, in order to provide an anti-skid control apparatus for a vehicle braking system which can be small-sized and light, and can minimize the possibility of locking of rear wheels, this applicant previously proposed an anti-skid control apparatus for a vehicle braking system which includes a fluid pressure control valve device arranged between a master cylinder and a wheel cylinder of a brake for the wheel and receiving control signals from a control unit measuring the skid condition of the wheel to control the brake fluid pressure to the wheel cylinder a fluid pressure control valve device was also provided for a pair of front wheels, consisting of a valve apparatus receiving fluid pressures from wheel cylinders of the front wheels. The valve apparatus was located between the pair of front wheels and a pair of rear wheels, and when any one of the fluid pressure control valve devices starts to control, the fluid pressure of the one of said rear wheels at the same side as the one front wheels in which the fluid pressure of the wheel cylinder of which is lower is controlled in accordance with the lower one of the fluid pressures of the wheel cylinders of the front wheels by the valve apparatus.
In the above-described anti-skid control apparatus, the control signals from the control units are formed by judging the skid conditions of the respective front wheels. On the assumption that the front and rear wheels are provided with tires of the same kind, the braking forces are so distributed to the wheels that the front wheels tend to lock sooner than the rear wheels, when the vehicle is rapidly braked on a road which is uniform in frictional coefficient.
However, when the above assumption is not fulfilled, for example, when only the front wheels are provided with spiked tires or chains for running on snow or ice, and rear wheels are provided with the normal tires, the rear wheels tend to lock sooner than the front wheels. In the above anti-skid control apparatus, the brake fluid pressure is not controlled with the locking of the rear wheel. When the brake fluid pressure of the front wheel is controlled over the limit locking pressure of the rear wheel, the rear wheel status locked, and the steering is unstable.
Even when the front and rear wheels are provided with similar tires, the rear wheel may tend to lock sooner than the front wheel, when the frictional coefficient of the brake lining becomes excessively low due to thermal fade in a front wheel brake apparatus and the limit lock pressure of the front wheel becomes excessively high, and particularly when the vehicle is rapidly braked on a road having a relatively high coefficient of friction. When a proportioning valve is used, the fluid pressure of the rear wheel is lower than that of the front wheel. However, this pressure increases in proportion to the fluid pressure of the front wheel, and may reach the limit lock pressure, causing steering instability.
Further, in order to provide an anti-skid control apparatus for a vehicle braking system which can be small-sized and light, and can minimize the danger of locking of rear wheels in any case, this applicant previously proposed an anti-skid control apparatus for a vehicle braking system with a pair of front wheels, and a pair of rear wheels. Wheel speed sensors were associated with the wheels, respectively. A first fluid pressure control valve device for controlling the brake fluid pressure of the wheel cylinder of one of said front wheels was arranged between a first fluid pressure generating chamber of a tandem master cylinder and the wheel cylinder of the one front wheel. A second fluid pressure control valve device for controlling the brake fluid pressure of the wheel cylinder of another of said front wheels was arranged between a second fluid pressure generating chamber of the tandem master cylinder and the wheel cylinder of the other front wheel. A control unit received outputs of the wheel speed sensors for measuring or judging the skid conditions of the front and rear wheels and for generating instructions for controlling the first and second fluid pressure control valve devices. A valve apparatus for generating a fluid pressure in accordance with the lower one of the brake fluid pressures of said front wheels controlled with said first and second fluid pressure control valve devices was arranged between the wheel cylinders of the front wheels and those of the rear wheels control unit discriminated between the frictionally lower one (designated as "low side") of the sides of the road on which the wheels are running on the basis of the measuring or judging results of the skid conditions of the rear wheels. It combined logically the measuring or judging results of the skid conditions of the rear wheels with the measuring or judging result of the skid condition of the one front wheel running on the low side to generate on instruction for controlling the first or second fluid pressure control valve device. It also generated an instruction for controlling the second or first fluid pressure control valve device, on the basis of the measuring or judging result of the skid condition of the other front wheel running on the high side (frictionally higher side) independently of those of the rear wheels.
In the above arrangement, when the valve apparatus operates normally, there is no problem. However, if the valve apparatus fails, the front and rear wheels remain connected in a diagonal relationship. Under a certain condition, one of the rear wheels locks, and the one front wheel on the same side as the locked rear wheel and the other rear wheel are subject to extreme "under braking" which means insufficient braking force.